For example, a battery pack used in an electric vehicle is structured in a state in which a pack casing is substantially tightly sealed in order to prevent an invasion of rainwater, dust, or so forth into an inside of the pack casing. In other words, in order to avoid a pressure variation within the pack casing involved in a charge-discharge, a temperature variation, or so forth, the inside and an outside of the pack casing is slightly communicated with each other through a, so-called, breathing hole or so forth which allows an entrance and exit of a relatively small quantity of air but basically the pack casing is in a tightly sealed state.
On the other hand, in a case where a large quantity of gas is abruptly generated in the inside of the battery pack due to an internal short-circuit of any one or more of batteries, it is necessary to allow a quick escape of the internal pressure of the battery pack. Patent document D1 and patent document D2 disclose kinds of pressure release valves in each of which an opening section is installed at a part of the battery pack and a lid member enclosing this opening section is eliminated when gas is generated. Patent document 1 discloses one of the pressure release valves in which the lid member enclosing a rectangular opening section is broken, deformed, or blown off in response to the pressure within the battery pack so that the opening section is released. Patent document 2 discloses the other of the pressure release valve in which the lid member is formed of a low melting point material and is melted due to heat of gas generated in the battery pack.
In the conventional pressure release valve type structures described above, the lid member enclosing the opening section is structured so as to be weak as compared with the battery pack itself. Thus, when gas is abruptly generated in the inside of the battery pack, the opening section is released due to a breakage of the lid member, a permanent deformation, a melting elimination, or so forth. After gas in the inside of the battery pack is approximately exhausted and the pressure within the inside of the battery pack is reduced, the opening section remains in the released state. Hence, an external air, viz., oxygen easily flows into the inside of the battery pack and a possibility that an abrupt oxidization reaction is developed in the inside of the battery pack occurs. It should be noted that a third embodiment of patent document 2 discloses the pressure release valve in which a reed valve type backflow prevention valve (check valve). However, this backflow prevention valve (check valve) serves to guide a flow of a high temperature gas to the lid member made of a low melting point material to the lid member and the lid member itself is melted at a time of the gas generation. After all, the opening section remains in the released state.
In addition, since, in the structures of patent documents 1, 2, a part of the battery pack, viz., the lid member enclosing the opening section provides a position at which a strength is low, such another problem that a countermeasure against stepping (flying) stones is needed newly and easily occurs.
Patent document 1: A Japanese Patent Application First (Laid-open) Publication No. 2014-041841;
Patent document 2;
A Japanese Patent Application First (Laid-open) Publication No. 2014-107178.